AVX Presses Case For Coaxial-Rotor Demonstrator

Can a small company pull off an upset and win a role in the most important rotorcraft demonstration of this decade? It may face competition from giants Bell Helicopter, Boeing and Sikorsky, but AVX Aircraft believes it can.

More than that, the Fort Worth company argues the U.S. Army, to be true to the intent of its Joint Multi-Role (JMR) program, should pick AVX for one of two high-speed rotorcraft demonstrators planned to fly in 2017.

The stakes are high. The JMR technology demonstration is a precursor to the planned Future Vertical Lift (FVL) Medium program to replace thousands of Sikorsky UH-60 Black Hawk and Boeing AH-64 Apache helicopters from the mid-2030s.

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It is a prize so large that Boeing and Sikorsky have teamed for JMR and FVL. They are offering a compound helicopter based on Sikorsky’s X2 high-speed coaxial-rotor configuration. Bell is offering a tiltrotor. Even EADS North America has put in a bid, likely based on Eurocopter’s X3 hybrid helicopter.

AVX’s design (see image) combines coaxial rotors with ducted fans and small wings. It is not the only small player with hopes of winning. Piasecki Aircraft has proposed a winged compound helicopter with a vectored-thrust ducted propeller, as flown on its X-49A SpeedHawk.

Established by former Bell employees, AVX was set up around the coaxial-rotor/ducted-fan concept. The company proposed the configuration to upgrade the Bell OH-58D Kiowa Warrior armed scout, but its breakthrough came when—with Bell, Boeing and Sikorsky—AVX won an Army contract to study concepts for JMR and FVL.

Coaxial rotors provide high hover efficiency and eliminate the power drain of a tailrotor, says Troy Gaffey, AVX president and chief engineer and former head of engineering at Bell. The ducted fans provide propulsion, so the rotors only provide lift, greatly reducing the power required. At the 230-kt. speed sought by the Army, two-thirds of power goes to the fans and a third to the rotors.

Not tilting the rotors to provide thrust reduces blade loads and cuts vibration by at least 50%. “We also fly with the fuselage a little nose-up for lower drag, so we get more speed with the same power,” Gaffey says. “The focus of the configuration is aerodynamic efficiency.” Ducted fans are smaller and lighter than open propulsors, with better cruise efficiency, he says.

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At maximum speed, only 60% of lift is from the rotors; the other 40% comes from small forward wings and the aft ducts and stub wings. But, at 230 kt., the Kamov-style rotors generate half the drag—as much as the airframe—so AVX is testing hub-and- mast fairings in a bid to reduce rotor drag by a third.

The high lifting capacity of the 56-ft.-dia. rotors allows the helicopter to carry significant external loads, Gaffey says. As proposed, the aircraft weighs 27,000 lb. carrying 12 troops and four crew, versus 22,000 lb. for the UH-60M. But it can lift 13,000 lb. externally, versus 9,000 lb. for the Black Hawk. “It is more like a Chinook from a cargo standpoint,” he says.

One source of the lifting capacity is the engine power needed to go fast. To fly 200 kt. with the basic 4,300-lb. payload would require 3,100-shp engines, but the target of at least 230 kt. would need more than 4,600 shp.

“In the hover, that’s a lot of extra power, but a large part of the Black Hawk’s mission is lifting heavy loads,” says Gaffey.

All the compound-helicopter candidates face the same problem. “The main issue with 230 kt. is the power in the aircraft. Bigger engines cost more and burn more fuel, but the side benefit is enormous lift margin,” he says. If the Army decides speed costs too much, AVX can remove the ducted fans. “Without fans, speed is in the 170-kt. range.”

AVX has assembled a large team of suppliers to build the aircraft, but why should the Army choose an outlier for such an important program? For Gaffey, it lies in the Army’s edict that JMR bidders must show a “compelling need” to fly a demonstrator to reduce risk for FVL.

“We don’t see any point in flying another tiltrotor. When there has already been the XV-15 and V-22, what is in Bell’s V-280 that needs to fly?” asks Gaffey. “It’s almost the same for the X2—they have flown it and it works. And the X3 has been flown by Eurocopter.

“In our case, the coaxial rotors and ducted fans need testing to verify the efficiencies,” he says. “The big manufacturers will probably continue on their own as long as FVL is on the horizon, so we are hoping the Army will keep the little guy in the program to provide a little competition.”

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